Multiplex signaling system



March 15 i927.

MULTIPLEX SIGNALING SYSTEM Filed March 12, 1925 5 Sheets-Sheet 1 Q 7 1 12; u m A 25% 53a ga m 33 3 T (IL N m m; M J F m 2 1 I B X 3 fl! 5 13 [mM M H. il? :5 7 AV. v 1 25 x m l J F w m ma A" u 4 I i w iii imam ELLAH1 A 7 .slil. L r m Iii m L ATTORNEY March 15 9 MULT I FLEX. SIGNALINGSYSTEM March 2 l 2.5 5 Shec t ATTORNEY March 15, 1927. ,735

H NYQUIST MULTIPLEX SIGNALING SYSTEM File d March 12, l925 5Sheets-Sheet 4 3%,: lNVENTOR flifigaw ATTORNEY March 15,1927. 1,520,735

H. NYQUIST v MULTIPLEX SIGNALING SYSTEM Fil d Ma h 12, 1,925 5Sheets-Sheet 5 17 16 if. 727 F7. 9

ATTORNEY Patented Mar. 15, 1927.

oans UNITED STTES PATENT FFICE.

HARRY NYQUIST, OF ILIILLB'URN, NEW? JERSEY. ASSIGNOR TO AMERICANTELEPHONE AND TELEGRAPH GUMEANY. A

rIULTIPLEX Application filed March 12, 1925.

l'he principal object of my invention is to provide a new and improvedmethod and suitable apparatus tor utilizin a line for the simultai'ieoustransmission or a number of mes-Rages. Another object of my invention toprovide for a stem ot multiplex telegraphy with discrimination bydifference of phase between messages in the same frequency range. Stillanother object of my in vention relates to providiiug for magnitudediscrimimition as well as for phase discrimination between differentmessages transmitted \vitl'iin the same frequency range. All otherobiect of my invention is to provide a component current of a certainfrequency to be transmitted for synchronizing purposes and also to beutilized as a signaling channel. Still another object of my invention isto provide for signaling on a synchronizing current component and toprovide means for con'ipcnsating at the receiving end so that thatcurrent may be delivered to a branch circuit for synchronizingunmodified by the signaling. These and various other objects of myinvention will become apparent on consideration of a limited number ofspecific examples of practice according to the invcntion. which I havechosen to illustrate and describe in this specification. Vt it-h theunderstanding that the following specification a disclosure of theseparticular examples of the invention and that the scope of the inventionwill be ind cated in the appended claims I now proceed to describe thestructure and operation of the apparatus shown in the drawings.

Figure 1 is a diagram ot multiplex sending equipment; Fig. 2 is adiagram of the corresponding receiving equipment; Fig. 3 is a diagran'iot a sending and receiving equipment with provision for automatic phaseadjustment; Fig. 4-. is a diagran'i of a station at the opposite end ofthe lines from the station of Fig. 3; Fig. 5 is a diagram of a detailmodification in Fig. 1; Figs. 6 and 7 are vector diagrams that will hereferred to in enplain m; operation of the apparatus shown in the otherigures; Fig. 8 is a modification corresponding to the upper part of Fig.2; F 0 is a diagran'i illustrating a revertshey that may be employed;Fig. 10 a diagram of circuits and a iiparatus for receiving me-sagestransmitted on a synchronizing cur rent and utilizing a derived doublefrequency CURPOBJ-KTIGN NEW YORK.

SIGNALIll'G-SYS'JEM.

Serial No. 15,058.

currentfor effecting the synchronizing; Fig. 11 is a diagram ofapparatus and circuits for receiving two messages on 90 phases of asynchronizing current and compensating ac cordingly in a branch circuitso that the syn-- chronizing current will be unaffected forsynchronizing purposes; Fig. 12 is a diagram corresponding to Fig. 11except that here four messages are involved, depending on phasediscrin'iination and also on magnitude discrimination, and Figs. 13 to18 are diagrams that will be referred to in explaining Fig. 11.

At the station West shown in Fig. 1, the constant speed motor M drivesthe inductor alternators, of which three are shown, G G and G ofdifferent carrier current frequencies. The output current from thegenerator G goes over the circuit 21-22-28 24; through an inductancecoil of impedance )jn, a resistance a and a condenser of impedance 7'nin series. where n has any suitable value, say 600 ohms. The value it isthe resistance of the generator Gr looking into it across the points 21,2t and itis important in the particular example illustrated that theimpedance of the generator should be substantially a pure resistance asseen from its output terminal. The network 21222324r is a balancedVVheatstone bridge because the product of the impedances of one pair ofopposite arms is equal to the corresponding product for the other pairof opposite arms. Hence there is no interference in the transformerwinding 37 due to electromotive force in the winding 38, nor vice versa.This is true for the particular frequency of the generator G It is alsotrue for other frequencies because with a variation of frequency theimpedance of the coil and the impedance of the condenser will varyinversely and, therefore, their product will. be constant, and equal toLooking across the circuit at in the direction of the arrow, theimpedance is n. this is true because the circuit of the coil 38 may beconsidered open on account of the bridge balance, and it follows thatthe impedance between the points 21 and 23 is heme-i in i -7' Thecurrents in the windings 37 and 38 are 90 degrees apart in phase. Toprove this, notice that if a given voltage is impressed across thepoints 21 and 23, the current into the generator will lead by 4E5degrees. The design is such that the im pedance across the terminals ofthe coil 23. is effectively a pure resistance. This being the case, itfollows that the current in the winding 37 due to a voltage froin thegenerator Gr will lead by degrees. Similarly, it can be shown that thecurrent due to the generator G in the winding 38 will lag 45 degrees,and hence the currents in the windings 37 and 38 are degrees apart.

Thus far it has been assumed that the resistance of the coil marked.with the impedance value +jn is negligible. Fig. 5 is a modified diagramto take care of the case when the resistance of this coil is notneglooted. Assume that its resistance is 7*) and associate another coilwith a close inductire coupling whose self-inductance is the same andwhose resistance is 9. Then with the modified impedance values writtenon Fig. 5,.it may readily be shown that the bridge is in balance asbefore.

To explain the operation of the keys K and K in Fig. 1, first assumethat the he and the resistances 31 and 32 are removed and that thepoints 27 and 39 are connected by a resistance equal to the resistance30. The points 28 and 39 will then be neutral points, no matter whatcurrent flows in the winding 37, and there will be no transmis sion. Itwill readily be seen that while the key K remains closed. the key K,gives zero resistance from 27 to 39 when it is closed for marking andinfinite re when it is open for spacing. As is well known, when twounequal resistances are joined it is permissible and often convenient tolook at the point of junction as a point of reflection and to considerthat there is a reflected current wave at the point of junction. In thecase here under consideration, the reflected wave from point 27 istransmitted in part into the line. lt is also well known that in thelimiting case when one of the resistances is made Zero and the other isleft finite the reflected current is equal to and in phase with theincident current, whereas in the limiting case when one of theresistances is made infinite and the other is made finite the reflectedcurrent is equal to and in phase opposition to the incident current. If,therefore, the impedance at is changed from zero to infinity the effectis to reverse the current transmitted to the line. Now if a resistanceartificial line is connected between point 9.7 and the point ofreflection and if this artificial. line is so designed that there is noreflection at point 27, then the incident wave will traverse the networkand be attenuated, moreover, the reflected wave will traverse the linealso. The

net result of the artificial line that the cur cnt transniiitted to thetransmission line is attenuated by twice the loss of the artificialline. While the function of the key K, is to open and close the circuitand thus to reverse the phase of the transmitted current, the functionof key K is to add or remove art ial line and thus to change themagnitude without changing the phase.

Thus it will be seen that the key l i, transmits by pha: reversal of thecarrier current and the key l1, transmits by changing the magnitude otone component of the carrier current m gtmerator In a similar manner,the keys K, and K respectively, transmit by phase reversal and magnitudechange in the other carrier current compoimnt which ditlers 90 degreesin phase from that controlled by the keys K, and la These two carriercurrent components oi; the same frequency, differing 90 (i grees inphase and varying in magnitude as determined by the operation of thekeys K and K are superposed in the input of the band filter F whoseoutput goes in multiple with the outputs from other similar filters tothe line Z These filters are confluent hand filters each with relativelynarrow band width and with the carrier frequency near the middle of itsrange so as to produce no serious phase shift during the building up ofthe signals. The currents to the line i V are amplified by the amplifierA The box ,1? represents a network similar to that betv. the gencratm'(l and the filter li Accordingly, four more messages will be carried inthe channel from the generator G through the network and the filter Fand added in multiple on the line Z Alternating current of anotherfrequency from the generator G, is passed through the network S, and thefilter F, and the terminal. repeater or amplifier A, to the line Thiscurrent, as a component on the line 7,, is the syi'ichronizingCUlUPODBDt for use at the re ceiving end, as will be explainediu'csently. It is also utilized for signaling by means of the sendingnetwork 3,, but for the present, the use of the network S, may bedisregarded and the current component from the gcnerie tor G, may bethought of transmitted for synchroni purposes only.

Referring to 2, this shows the receiving station which may be calledEast. The currents coming in over the line Z, pass through theadjustable artificial line or net work l-l which. is controlledautomatically in a manner to be described presently. This network T lattenuates the currents so as to give a uniform magnitude in its outputfor the same signal elements from one time to another. The currents,thus attenuated to a standard magnitude, pass through the arm pliiier orterminal repeater A, to the filters l? and F in multiple. These filtersSit) correspond in their frequency ranges, respectively, with thefilters F F and F, at station West.

The current of pilot frcipiency tor syn chronizing goes through the[ilter F to branch circuits: one oi these hranch circuits goes to areceiving; network ll and another to a network X whose tunction will he9X- plaincd presently. l i hatever the modification hy signaling to theinput of the network X, its output (as will he explained) is a simple.unmodified. alternating: current 01 he quency definitely related to thefrequency of the generator G, at the senijling' end, and this outputcurrent is applied to drive the synchronous motor SM, which in turndrives three generators G G and P, of the same frequency respectively51,, G and G, at station West. The network X takes different forms as inFigs. 1O, 11 and 12, and a switch is provided which should he positioned accordingly. This switch S connects the input of amplifier 1 torec ive an alternating: electromotive :lorce of nearly coi magnitude. aswill he shown in each. instance.

The voltmeter relay V is connected across the output from the amplifierA, and normally holds its index it a certain, position corrospondino toa certain voltage. lVlu-uiever this 'r'OltZtQ'fi changes a little eitherway, the index closes one or the other of the circuits tor the magnetsteland 4t? causing the armature 4:3 to move and bring the wheel il intocontact with one side or the other oil the yoke 42. The wheel ll. isrotated continw ouslv hy the motor M, so that the artificial line N isadjusted one way or the other to adjust the attenuation therethrough.Thus it will be seen that by means of the voltmeter relay V and the asociated apparatus, the current of pilot frequency is kept at uniiormvoltage delivered to the input of the arm pliticr A This insures thatfor the same signal elements in the other channels, the voltagesdelivered to the input of the an1pli tier A, will he unitornl. Even ifthe attenuation on the line Z, varies from time to time, may he the casedue to weather conditions. etc, there will he no corrcs onding ariationot the current and volta, 'e magnitudes in the output from the network Nat station East. lit the received currents at East "ere to any from timeto time. it might happen that sigrnals ot' low 1 nitudc would sometimeshe strongr cnou i w do h to atl'ect the hio h magnitude receiving:apparatus, or that hich ia rnitude would sometimes he too weak to a'tlect the high magnitude receivi a niai-atus. By the a ljustrient at l i.such outcome prevented.

The i'uodulated on out oi carrier tretguency deteruunod hy the generator(-2, at

station lVcst will. accordingly. lind its through the .tilter l 50 tothe primary of the transiorincr whose motive forces across theresistance n and the condenser will he 90 degrees apart. Thus there willhe applied to the grids of the dctectors D and D locally generatedelectromotive force from the terminals of the resistance n of the sameor opposite phase with the received carrier current component controlled hy keys K, and 13. and there will he applied to the grids of thedetect D, and locally generated electroniotive torcc from the terminalsof the condenser whose impedance value is in, ot the same phase as thereceived carrier current component contr lled by the keys K, and IQ. Theelectroinotive forces due to these received. carrii-n' currentcomponents will he superposed in the secondary windings 55F 'i and 5f i'on the locally generated electroniotive tort-e.

Let the voltage on the grids of tubes D and due to the generator G, herepresented hy the vector OX in Fig. 6. The superposed electromotivetorce due to the received. currents on the line will comprise twocomponents 90 apart. One of these will he 012 when both keys K and K aremarl:- ing 7 012 when key K, is marking and key K is spacing,

012 when key K, is spacing is marking,

012 when both keys are spacinp'. (As will he seen. unprimed numerals areused wh n the corresponding detectors are marking, primed when they arespacing"). Similarly, keys l1, and K. will determine a component O-34.-,0-3l, O-3 l or 034E. These two components, combined with the locallygenerated electromotive torce OX give one of 16 diiierent resultantsfrom 0 to a point which may readily be identified hy the notation on 6.For example, when all four keys are marking the resultant is the vector"from O to the point 1.3%; for another example, when all four keys arespacing the resultant is the vector from O to the point 1.23-i. It willhe soon that tour substantially ditl crent vector mannitudcs may he puton each of the detectors ll. and D 'lhese may he classed as lying in thetour zones 12, 12, 1'2 and 1'2, as shown in Fig. 6. Detector D, and itsassoand key K lUU llO

till

ciated elements of apparatus are adjusted so that when the grid voltagehas some value between the genes 1'2 and 12, the core of the polar relayPR will be. substantially decnergized. But when key K is marking,whatever the condition oi? the other keys K K and K the resultantelectromotive 'torce on the grid of detector D will have a magnitudethan OK and the polar relay PR controlled by detector D will be on themarking side. On the other hand. when key K spacing, whatever thecondition ot the other three keys. the resultant elcctron'iotive torceon the grid of detector D will be ct in nitude lGLS than ()1: and therelay Pltl. will be on the spacing side.

The relay NR is a neutral relay and is adjusted to be marginal in itsoperation. The adjustments of this relay NR and the associated detectorD. and their circuits are such that the relay NR is deenergized when thevoltage on the grid oi the corresponding detector D has the magnitude OKin Fig. 6. When the magnitude of this voltage changes either way, themagnetism in the core of relay NR departs correspondingly one way or theother from zero. Because of its marginal character the relay NR will notoperate for a change of voltage to zone 12 or to zone 12, but it willoperate for a change to zone 12 or zone 12. lts armature is made slowenoiiigh in release so that on a reversal of the magnetism in the coreit will not drop; in other words. the armature of the relay NR will. notdrop for a change .in the my K by which the grid voltage goes from oneto the other of zones 12 and 1'2.

To recapitulate briefly. key K causes shifts between zones 12 and 1'2 onthe one hand and Zones 12 and 1'2 on the other hand and actuates polarrelay PE, accordingly, whereas key IQ. causes shifts between zones 12and 12 on the one hand and zones 1'2 and 1'2 on the other hand, andoperates neutral relay NR accordingly.

The operation of detectors D and and their corresponding relays isclosely similar to that described for detectors D and l). and theirrelays and need not be repeated in detail.

Fig. 8 shows a modification of the part of Fig. 2 associated with filterF. and generator G In this case the functions of the pair of detectors Dand D are performed by the single detect-or D and similarly D takes theplace of D. and D Thus far, the system of Figs. 1 and 2 has beendescribed in the main as it the component current oi trequencydetermined by the generator G at Vtest went through the network X atEast and was applied tor driving the synchronous motor SM. But at lVesta sending network S is shown.

This may be a simple reversing key, as shown in F 9, and its etl'ectwillbe merely to reverse the phase of the synchronizing current. Thecorresponding apparatus indicated by the boxes R and X in Fig. 2. isshown in detail in Fig. 10. The received current component ofsynchronizing frequency passes through the filter F (Fig. 2) and overthe conductors 71 to the push-pull detector 72-73. The output from thisdetector is taken oil through the transformer T-l. which is so locatedin the system that the fundamental and the odd harmonics are suppressed.r rccordingly. the electromotive l'orce induced in the secondary ot thetransformer 7%- has a fundamental of double frequency. It will beapparent that the phase of this current is not affected by reversals otthe input current. The coil 75 and condenser T6 are combined to be tunedto this double frequency. so that it is passed to the energizing windingot the synchronous m0- tor SM.

The current through switch S to voltmeter V of substantially constantmagnitude, because the phase reversals do not affect the magnitude.

The synchronizing current, inodilied by phase reversals for signaling.goes through the potentiometer 50 (Fig. 10) to the detector 1),. whoseoutput contiols the polar relay PR This is similar to the correspondiugl v designated part o'l Fig. and need not he described here indetail.

In order to transmit two messages on the synchronizing currentconipoi'ient. each by phase reversals of respective cou'ipouents apart.the sending network of Fig. 1 may be employed at with the understandingthat. the keys K and K are not employed. In this case the elements ofFig. 2 represented by the boxes ll, and X. will be as shown in detail inFig. 11. The branch conductors 71 lead to a delay network Y. As will beseen presently. the network to the right of Y is controlled by thereceiving relays and the object of the delay network Y is to introducethe same delay as tor the operation oi these relays.

Here. as for Fig. 10, the current through switch S5 to voltmeter V isot' substantially constant magnitude because the phase shifts forsignaling do not a'l'iect the i'uagnitude.

The receiving network at the upper part of Fig. 11 is the same as thatportion of the upper part of Fig. 2 obtained by omitting the portionsinvolved in. magnitude discrimination. It will be seen that in Fig. 11an extra contact has been added on each of the two polar relays PR, andPR. lVhen a. phase reversal on one component of the synchronini gcurrent actuutes the polar relay PR the contact 8% is actuated and thiscon trols the relay 83 whose contact 82 reverses the electromotive forceon the primary 77 of weaves the transformer 77-78. Every reversal ofphase of the component for the detector D. leads to such a change in thenetwork at the lower part of Fig. 11 to produce a compensating reversalthere. Similarly, every reversal of phase or component for the detectorD, produces a reversal tor the terminals of the primary 77 of thetransformer 77-78. Accordingly, the amplifier 85 receives only theunmodified alternating cur-- rent of synchronizing frequency and itsoutput goes to drive the synchronous motor SM.

Referring to Fig. 13. let (iii represent the current received throughthe network Y roin the line. OL is made up ot the two components (litand DB, due to the transmission of signals, the synchronizing channelbeing used in this case for two message channels operated on theprinciple of phase discrimination. The electromotive force betweenpoints 80 and 81. across the inductance in and resistance a in thenetwork leads the cur rent by degrees and may be represented by 01. Fig.14L. OC, equal in length to ()1, represents the electromotive forcebetween the points 80 and 81 across the condenser and resistance in thisnetwork. C ne-halt of each of these electromotive forces is applied tothe respective primaries of transformers 77 and 77, and consequently theresultant of these potentials is applied to the grid of tube 85. This isshown in F 15, 0G representing the grid potential whose components arethe equal magnitude potentials OT and 0C. The phase of the synchronizingfrequency is determined by the phase of 0G. The output of tube 85remains constant in magnitude, of course, inasmuch as the mag nitude of()L remains constant regardless of the signals being transmitted.

The purpose of the apparatus under discussion, then, is to keep 0Gconstant in phase as the phase of OL is changed due to signaling. Thefollowing example will show that this is true for one specific change inthe phase of 0L, and it can be similarly shown that this also holds forthe other pos sible cases.-

Referring for a moment to Fig. 11, it should be understood that areversal of 0A (Fig. 13) operates relay PR which in turn operates relay83 and so reverses the phase applied to transformer primary 77.Similarly, a reversal of OB reverses the phase ap plied to 77. It is thepurpose of delay networlr Y to cause the reversal in 77 or 77' to occurat the same instant that UL changes.

N ow as a specific case, suppose that GA is reversed changing 0L to 0Las shown in F ig. l6. OI then becomes 01 Fig. 17, and OC becomes 0C Dueto the operation of the relays as explained above, 0C, reversed isapplied to 77 so that GO becomes 00/, Fig. 18. OI becomes 01 in phasewith 01".. 0G,, the resultant of 01 and 00 is however the same in phaseas 0G.

A similar result can be obtained by following through a reversal of OBor reversals in Oil. and OB together. Consequently, it is evident thatthe phase and magnitude of the synchronizing frequency are maintainedconstant.

To transmit tour messages on the synchronizing channel, utilizing bothphase discrimination and magnitude discrimination, the netw S of stationlVest may be made like the sending network shown in the upper part ofFig. 1. In this case the apparatus represented by the boxes R and X of sion East will be as shown in detail in The message-receiving networkshown in the upper partof Fig. 12 is the same as in the upper part ofFig. 2 except that each of the tour receiving relays PR PR NR, and hasan extra contact controlling the respective relays 94, 95, 96 and 9?,whose function will be explained presently.

Tl'ie incoming synchronizing current modified both by phase reversalsand magnitude changes goes through the relay network Y, whose purpose isthe same as for Fig. 11. The electromotive force at the output terminalsof the delay network Y may be represented by the expression A cos pt-l-Bsin pt where A and B may each have one of two magnitudes and may each besubject to reversal of sign.

assume for the present that the resistance a, b is shunted, as indeed isthe case, with the contacts in the positions shown in Fig. 12.

It will be seen that there are two parallel branches from b to 0, onecomprising inductance L in series with resistance R, and the othercomprising capacity C in series with resistance B. These quantities arerelated by the equation 1 2 vrfL R m Accordingly, it follows that thecurrent through the inductive branch lags and may be represented by theexpression g -(cos pt-lsin pt)+% sin ptcos pt) Also, the current in thecapacity branch leads and 1s %(cos ptsin pt sin pt+ cos 392,)

It will be seen that the conductors 86 take off current from adjustabletaps along the resistances It in the two branches between 6 and 0. Letthe middle point of each resistcos 193+ sin 110+ (sin ]3t cos pt) I Ifiz cos pt sin pt) G P 0081375) which may be writ-ten mA mB +mA m B) cospt It is a determinate problem to choose m and m corresponding torespective values of A and B in such a way that the value of thisexpression shall not be changed, in other words, so that the voltage inthe conductors 86 shall be equal to S cos pH-T sin 7225 where S and Tare constants.

Equating the corresponding co-eiiicients of the last two expressions andsolving, and simplifying by putting S:T:1/2 we get These equations givethe following table of Values for typical values of A and B:

As shown in Fig. 12, the potentiolneters, each having a total resistanceR, are operated by the four relays 94-, 95, 9t; and 97. It will be seenthat relay S t reverses the algebraic sign of m and the relay 95reverses the algebraic sign of m. These two relays are controlledrespectively by the receiving relays PR and PP so that they eii'ectreversal of sign of m and m according as the signs of A and B arereversed respectively.

Another relay 96 changes the magnitude of m and also controls theresistance a, Z). This relay is 'overned by the receiving reweaves layNR and is operated by changes of magnitude in A. As will be explained alittle more definitely a few lines further along, this relay 96compensates for changes in A so as to keep the magnitude in the outputconductors 8G unchanged. Similarly, the relay 9? changes the magnitudeof m and also controls the resistance a, 7), and is itself controlled bythe receiving relay NR By proportioning the resistance a, b so that itspresence reduces the current in the potentiometer in the ratio of 3 to5, the above conditions for m and m to keep the voltage constant in theconductors 86 are satisfied, provided the taps along the resistance R atthe left are arranged so that in this specific instance and similarlyfor the other resistance R at the right.

Thus it is established that the voltage impressed on the inputtransformer for the amplifier 87 remains constant in magnitude anduninterrupted in phase regardless of the variations in voltage of thecurrents received from the line as caused by the signals on the line. Asin. the case of Fig. 11, the delay network Y must be so adjusted thatany change in the line voltage arrives at a and c at the instant thatthe proper relays in the potentiometer have operated to care for thischange.

The conductors 88 and switch S apply a constant alternatingelectromotive force to amplifier A whose output goes to voltmeter V.

By means of the apparatus of Fig. 12 it becomes possible to synchronizeover a channel that carries tour messages by means of phase andmagnitude discrimination.

The system of Figs. 1 and 2 is adapted for operation. in a case in whichthe phase shift involved in transmission over the line Z issubstantially the same from time to time. This being the case,adjustment made by the taps 53 and 54: will be lasting and will not haveto be changed from time to time. In case the properties of the line aresuch that the phase shift over the line changes from time to time, thesystem shown in Figs. 3 and t may be employed. At station lVest shown inFig. 3, the corn stant speed motor M the generators G G and G theadjustable network N, the sending networks S and S. and the respectivefilters F F and F are the same as for Fig. l. interposed between eachgenerator G G or G and the respective network N, S or is an adjustablephase shifter P P or P In the system of Figs. 3 and 4, two lines areemployed, Z for transmitting from west to east and Z for transmittingfrom east to west. These may be the wires of a four-wire lll) Iii

- in the lower part of Fig. 2.

system. Reception at the station test on the line Z is the same asexplained heretofore for Fig. 2, and the upper part of Fig. at is thesame as Fig. 2.

The branch circuits from the generators G G, and go, respectively, tothe sending networks S S, and S and the respective filters F and F tothe line Z from east to west. Thus it will be seen that the ap aaratusat the lower part of Fig. 41- is very similar to that of Fig. 1, thepilot carrier frequency current being that received from station lVeston the line Z and sent back over line Z At station lVest shown in Fig.3, the incoming currents on the line Z are adjusted automatically formagnitude in the adjustable network N by the voltmeter relay V andassociated apparatus like that shown The received pilot frequencycurrent its signal variations neutralized in network X, and goes thenceto the voltmeter relay V, and a iultiple branch carries this samefrequency to the primary of the transformer 61 whose secondary isconnected apply the result ing electromotive forces alike to the gridsof the two three-electrode vacuum tube detectors 62 and 63.

Reception in network R, similar to that in networks and B5. The outputfrom the pilot frequency G, goes to network R, from a branch circuitthrough the phase shifter P and the same current also goes to theprimary of the transformer 6t and develops an electromotive force whichis applied in phases opposite to each other to the grids of the twothree-electrode vacuum tube detectors 62 and 63. The normal adjustmentof the phase shifters l and P, will be such that the electromotiveforces developed in the secondaries of the transformers 61 and 645 willbe 90 degrees apart. Accordingly, under such normal conditions, the fullline vector diagram of 7 will apply. On one grid, the electroinotiveforce will be OY, the sum of OK and KY. On the other grid, it will beOY, the sum of OX and XY.

But suppose there is a slight departure from the 90 degree relationbetween the two electromotive forces in the secondaries of thetransformers 61 and 64:. The vector diagram shown in dotted lines in 6will then obtain and the electromotive forces on the grids of the twotubes 62 and will become, respectively, OZ and OZ.

l Vhen the electromotive forces on these girds were equal, namely OY andOY, the output currents of the detectors 62 and 63 were equal and hencethe armature of the relay 65 stood at neutral. But with the unequalelectrornotive forces OZ and OZ on the grids of the two tubes, theoutput currents are unequal and the ar nature of the re lay 65 willclose on one contact or the otl er, energizing one or the other of themagnets 66 and 67. The result will be to push the wheel 68 against oneside or the other of the yoke 69. This wheel 68 is turned constantly bythe motor hi so that the cams 70 are shifted to the ri 'ht or the left.These cams engage respective rollers which are mechanically connected sothat their dis-- placements adjust the phase shifters P,, P P P P and FThe adj stiuent of the phase shifter P, and P, restores the degreerelation. between the electromotivc forces in the secondaries of thetransformers 61 and The adjustments are opposite and equal for P, and Plikewise for P and P and likewise for P, and P and in each of the threeround trip channels of different carrier current frequency, the currentsare made to arrive at the receiving apparatus in proper phase relationwith the locally generated currents.

I claim:

1. The method of si naling which consists in sending a current conpringa component of a certain frequency modified at the sendin one accordingto a signal to be transmited, and detectin" such signal at the receiviugend and a. compensating the received. component in a branch circuitaccording to the detected signals to remove the signal modificationstherefrom, and applying the co'upcnsated current to synchronize the receiving apparatus with the transmitting apparatus at the other end ofthe line.

2. 1n the art of signaling electric currents, the method which consistsin signalon a synchronising current and at the receiving end detectingthe signals and also deriving a current of pure frequency to be appliedfor synchronizing purposes.

In the art of transmitting messages by means of electric currents, themethod which consists in signaling on a synchronizing component ofcurrent and at the receiving end detecting the signals and compensatingthe received component in a branch circuit accordingly to make it ofpure frequency and applying it for synchronizing purposes.

l. In a system for signaling by means of electric currents, receivingapparatus comprising two multiple branch circuits to receive a currentcomponent of a certain frequency, apparatus associated with one suchbranch circuit to detect a message carried by such component, andapparatus in the other branch circuit to derive a current of purefrequency unaffected by the message and to apply such current forsynchronizing the receiving apparatus.

5. In a signaling system, receiving ap paratus comprising a filteradapted to pass a current component of a certain frequency llll) and twomultiple branch circuits connected with the output (Iii said ti tcr,appa: itus in one such branch to detect a message on the said componentof current. apparatus in the other said branch to derive a current ofpure frequency unmodified by the messag and means to apply the lastmentioned current. for synchronizing purposes.

6. In a signaling; system, receiving apparatus comprising a filteradapted to pass a current component ot a certain frequency and twomultiple branch circuits connected vith the output of said filter,apparatus in one such branch to detect a message on the said componentof current apparatus in the other said branch to compensate for thesignal modifications. control relays therefor governed by the apparatusin the receiving branch, and synchronizing apparatus to be operated bythecompensated current.

7. In a multiple): carrier current system. means for generating carriercurrents o't various frequencies and super-posing them upon atransmission line, means to modulate all of these carrier currents in.accordance with messages to be transmitted. and synchronizing means tobe controlled by one of these carrier currents.

8. In a system for signaling by means oi electric currents, receivingapparatus coinprising two multiple branch circuits to re ccive a currentcomponent of a certain frequency, apparatus associated with one suchbranch circuit to detect a message carried by such component, apparatusin the other branch circuit to derive a current of pure frequencyunaiiected by the message and to apply such current for synchronizingthe weaves receiving apparatus, and a delay network in the lastmentioned branch circuit.

S). in a s' *ialing system, receiving apparatus comprising; a lilteradapted to pass a current component of a certain frequency and twomultiple branch circuits connected with the output oi. said filter,apparatus in one such branch. to detect a message on the i-filitlC(H'HPOIlQHiZ oi current, apparatus in the other said branch tocompensate for the signal n'iodilicutions, control relays there-torgoverned by the apparatus in the receiving, branch, synchronizingapparatus to be perated by the compensated current, and a delay networkin the said other branch to equalize the operation of said relays.

10. The method of signaling. which cons ts in sending a currentcomprising a component ot' a certain frequency modified at the sendingend according to a signal. to be transmitted, and at the receiving end.detect ing' the signal and also deriving from said component anunmodified current of definite frequency, and applying it for phaseregulation or the system involved.

11. In the art of signaling by electric currents, the method whichconsists in signaling on a synchronizing current, and at the receivingend det cting the signals and also deriving; a current ot pure frequencyto be applied for compensation of variations of the propagation constantinvolved in transmission.

In testimony whereof, I have signed my 1211119 to this specificationthis th day of Iiiarch 1925.

HARRY NYQUIST.

